DNA Replication, Transcription and Translation

DNA Replication
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 DNA undergoes what is known as semi conservative mode of replication wherein the daughter DNA contains one DNA strand of the parent. The replication can be explained in the following stages.

Unwinding of parental DNA
The supercoiled duplex DNA unwinds with the aid of several proteins. DNA helicase promotes the unwinding by binding to the single-stranded DNA. This followed by the negative supercoiling of DNA by DNA gyrase which lowers the the energy required to disrupt a base pair. Another helicase called the rep protein also helps in the denaturation of DNA by disrupting the base pairs.

Synthesis of an Oligonucleotide primer
The unwinding of DNA is followed by the binding of protein dnaB at the origin of replication of DNA and remains bound to the replication fork. This binding acts as a signal for a protein called primase an RNA polymerase which synthesizes a short complementary strand at the origin of replication.

Discontinuous synthesis at the Replication fork
The elongation of the primer on the template strands is done by an enzyme known as DNA polymerase. Synthesis of complementary DNA occurs simultaneously on both the parent strands. However the synthesis occurs from 5'-3' continuously on only one strand called the leading strand. On the other strand known as lagging strand chain growth occurs discontinuously. Short strands of polynucleotides known as Okazaki fragments are formed complementary to the lagging strand.

Primer Excision and Phosphodiester bond formation
In this step a ribonuclease removes the RNA primer, the DNA polymerase fills the gap and DNA ligase fills the nicks between the DNA fragments.

Transcription and Translation
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The expression of genes into proteins and is a process involving two stages called transcription and translation. In the transcription stage a strand of DNA molecule serves as a template for the synthesis of an RNA molecule called messenger RNA. This messenger RNA is then translated into proteins on ribosomes. The transcription step that involves the synthesis of mRNA from can explained in the following stages.

RNA polymerase binding to specific site on double-helical DNA
The RNA polymerase of prokaryotes is an oligomer composed of fice subunits arranged in an oligomer known as core enzyme. A sixth subunit called sigma subunit binds to core enzyme to give the holoenzyme. This enyme is responsible for binding RNA polymerase holoenzyme to specific transcritption sites on DNA. In eukaryotes there are three types of RNA polymerases called RNA polymerase 1,2 and 3 which synthesize different types of RNA.

Synthesis of RNA
There are many binding sites on DNA for RNA polymerase. Binding at certain sites is stronger than the rest and causes the initiation of RNA synthesis. These sites are known as promoter regions. In general these regions contain the following sequence

5'TATPuATG3' 3'ATAPyTAC5'

The binding is followed by the separation of DNA strands and synthesis of RNA by RNA polymerase.

Termination of Transcription
Specific DNA sequences rich in T followed by G-C rich sequences are responsible for termination of transcription. When RNA polymerase reaches these T rich sequences it pauses and the synthesized RNA is released. The termination process is aided by a protein called rho factor. It binds to RNA polymerase, RNA and DNA at the pause signal and RNA is released.

Additional Resources
For additional information, see: DNA Replication, Repair, and Recombination

For additional information, see: Nucleic Acids

For additional information, see: Translation